1. Field of the Invention
This invention relates to an image reading apparatus and an image reading method which scan the reading face of a document optically and convert the scanned data into color image data or monochromatic image data.
2. Description of the Related Art
In a conventional image reading apparatus (or a color image reading apparatus) which reads images on a document in color, the entire document is scanned optically by moving the carriage and the color images are read by the color CCD sensor. The conventional color image reading apparatus uses a 3-line CCD sensor as a color CCD sensor. The 3-line CCD sensor is composed of the following three line CCD sensors: a first line CCD sensor that outputs a red component (R signal), a second line CCD sensor that outputs a green component (G signal), and a third line CCD sensor that outputs a blue component (B signal). In an image reading apparatus with three line CCD sensors, when a monochromatic image is read, a monochromatic image is created on the basis of the signals (RGB signals) outputted from the three line sensors.
In the conventional image reading apparatus, the output of each of the three line CCD sensors constituting the 3-line CCD sensor is not always uniform. This nonuniformity results from variations in the sensitivity of a pixel unit in each line CCD sensor and a decrease in the amount of light around the lens caused by the light distribution characteristic of the exposure lamp that illuminates the document and the characteristic of the lens. Thus, in the conventional image reading apparatus, a reference plate (shading correction plate) acting as a white reference is read by the 3-line CCD sensor. On the basis of the result of the reading, the output signal from the 3-line CCD sensor is corrected (or the shading is corrected).
Therefore, in the image reading apparatus with the 3-line CCD sensor, the white reference plate is always read in the same reading position. This is because the image reading apparatus with the 3-line CCD sensor reads not only color images but also monochromatic images with the three line CCD sensors. Consequently, the conventional image reading apparatus with the three-line CCD sensor reads the shading correction plate in the fixed reading position, regardless of whether the image of the document is read in color or in monochrome, and makes a shading correction of the output signal from each line sensor.